linux-kernel - [PATCH 2/2] iommu/vt-d: Remove hardware automatic ATS dependency

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Date: Mon, 24 Jun 2024 13:25:01 +0800
From: Lu Baolu <baolu.lu@...ux.intel.com>
To: Joerg Roedel <joro@...tes.org>,
	Kevin Tian <kevin.tian@...el.com>,
	Yi Liu <yi.l.liu@...el.com>,
	Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc: Will Deacon <will@...nel.org>,
	Robin Murphy <robin.murphy@....com>,
	iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org,
	Lu Baolu <baolu.lu@...ux.intel.com>
Subject: [PATCH 2/2] iommu/vt-d: Remove hardware automatic ATS dependency

If a device is listed in the SATC table with ATC_REQUIRED flag set, it
indicates that the device has a functional requirement to enable its ATC
(via the ATS capability) for device operation. However, when IOMMU is
running in the legacy mode, ATS could be automatically supported by the
hardware so that the OS has no need to support the ATS functionality.

This is a backward compatibility feature which enables older OSs. Since
Linux VT-d implementation has already supported ATS features for a long
time, there is no need to rely on this compatibility hardware. Remove it
to make the driver future-proof.

Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
---
 drivers/iommu/intel/iommu.c | 9 +--------
 1 file changed, 1 insertion(+), 8 deletions(-)

diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
index 07e394dfccc1..b63347c8bf5d 100644
--- a/drivers/iommu/intel/iommu.c
+++ b/drivers/iommu/intel/iommu.c
@@ -3056,14 +3056,7 @@ static bool dmar_ats_supported(struct pci_dev *dev, struct intel_iommu *iommu)
 	dev = pci_physfn(dev);
 	satcu = dmar_find_matched_satc_unit(dev);
 	if (satcu)
-		/*
-		 * This device supports ATS as it is in SATC table.
-		 * When IOMMU is in legacy mode, enabling ATS is done
-		 * automatically by HW for the device that requires
-		 * ATS, hence OS should not enable this device ATS
-		 * to avoid duplicated TLB invalidation.
-		 */
-		return !(satcu->atc_required && !sm_supported(iommu));
+		return true;
 
 	for (bus = dev->bus; bus; bus = bus->parent) {
 		bridge = bus->self;
-- 
2.34.1